1. Technical Field
The present invention relates to an oscillation circuit, a semiconductor integrated circuit device, a vibrating device, an electronic apparatus, a moving object, and the like.
2. Related Art
In order to make a frequency of an oscillation circuit variable, a method in which a voltage is applied to a variable capacitive element arranged in the oscillation circuit to change capacitance is known. An oscillator which controls a frequency with voltage is generally called a voltage controlled X′tal oscillator (VCXO). As an oscillator in which a frequency deviation is suppressed with respect to temperature using this principle, a temperature compensated X′tal oscillator (TCXO) is known.
In a discrete oscillator, although a variable capacitive element having a large change in capacitance is selected to constitute a circuit, when implementing an oscillation circuit in the form of an integrated circuit, the characteristics of usable variable capacitive elements are limited. That is, if an oscillation circuit is implemented in the form of an integrated circuit, in general, a variable capacitive element has a smaller change in capacitance than a discrete variable capacitive element. This is because a dedicated process is required in order to obtain a large change in capacitance.
In recent years, reduction in size of a crystal oscillator is demanded, and implementation of an oscillation circuit in the form of an integrated circuit is in progress. However, when an integrated circuit is used, the variable amount of a usable variable capacitive element is limited, and thus there is a problem in that a necessary frequency variable width or linearity is not obtained. In other words, there is a problem in that it is not possible to appropriately obtain the sensitivity characteristic of the variable capacitance element.
In JP-A-2007-19565, a single control voltage which is applied to two variable capacitive elements is divided halfway by two level shift circuits, given a potential difference, and then applied to the variable capacitive elements. At this time, a configuration is made such that the C-V characteristic of one variable capacitive element is linear in a region lower than the center voltage of the control voltage, and the C-V characteristic of the other variable capacitive element is linear in a region higher than the center voltage of the control voltage. For this reason, it is possible to allow the control voltage to fluctuate in a wider range than the related art while securing linearity and to obtain a necessary frequency variable width.
However, in JP-A-2007-19565, a plurality of level shift circuits are required for each control voltage. For example, in the TCXO, a plurality of kinds of control voltages are used in order to control frequency. At this time, level shift circuits corresponding to the number obtained by multiplying the number of kinds are required. Accordingly, when the method disclosed in JP-A-2007-19565 is applied to the TCXO, circuit scale and power consumption tend to increase.